The present invention relates to electronic attenuators and, more particularly, to a circuit and method for producing a control signal proportional to absolute temperature for controlling the attenuation of such circuits.
The automatic control of attenuator circuits is frequently required in the design of telephones and other audio products. For example, contemporary speakerphones are typically operated in a half-duplex mode of operation wherein transmission and reception of speech audio signals is not permitted simultaneously. Hence, at any particular time, the speakerphone is either in a transmit mode, idle mode or receive mode as is well understood. In order to provide the required half-duplex operation the speakerphone includes a pair of attenuators, a transmit attenuator circuit placed in the transmit signal path of the speakerphone and a receive attenuator circuit in the receive signal path thereof. The transmit and receive attenuators are controlled by a single circuit and are complementary in function, i.e., in response to a control signal supplied from the control circuit, one is operated at maximum gain while the other is operated at maximum attenuation and vice versa. The two attenuators are never both on or both off. Thus, for instance, if the user is speaking, the speakerphone is placed in the transmit mode wherein the transmit attenuator is operated at maximum gain while the receive attenuator is operated at maximum attenuation to inhibit a receive signal from adversely affecting the transmission of the speech signal from the user. Similarly, in response to a controlling received audio signal the receive attenuator is automatically switched to maximum gain while the transmit attenuator is operated at maximum attenuation. In the idle mode, whenever audio signals are neither being transmitted or received, the gains of both the transmit and receive channels are reduced to some mid value thereof.
In at least one prior art speakerphone the transmit and receive attenuators, as well as, the attenuator control circuit, are manufactured in integrated circuit form. The transmit and receive attenuators in this prior art speakerphone are actually a pair of gain controlled two or four quadrant multiplier circuits which are well known to those skilled in the art. The two attenuators have a commonly connected gain control input to which the gain control signal is applied. The gain control inputs of the two multiplier circuits are such that, in the absence of a gain control signal, the transmit attenuator is operated at maximum gain while the receive attenuator is operated at maximum attenuation.
A problem arises due to the temperature dependency of the multiplier circuits, i.e., the gain of the transmit and receive attenuators varies as a function of temperature. This is undesirable as the gains of each attenuator should remain constant in order to provide high quality speakerphone operation.
At least one manufacturer, in an attempt to provide temperature compensation, uses an external thermistor to produce a current internally to the integrated speakerphone circuit which has a predetermined temperature coefficient. This internally generated current produces a control voltage that drives the two attenuators. In this manner the control voltage has a temperature characteristic which approximates the gain verses temperature characteristics of the two attenuators. However, a problem exists with this temperature compensation scheme in that the control voltage does not precisely track the variations and the gain of the attenuators over temperature because the temperature characteristic of the external thermistor does not track that of the integrated circuit attenuator circuits. This can produce an undesirable offset between the relative gains of each of the two attenuators whereby the log gains thereof do not remain constant.
Hence, there exists a need for an attenuator control circuit that produces a control voltage at an output that is proportional to absolute temperature which can be used to control the gain of integrated circuit attenuators of the type described above.